1. Field of the Invention
The present invention relates to a toroidal continuously variable transmission that performs a continuous or stepless speed change by changing a tilt angle of power rollers places in abutment against both an input disk and an output disk arranged opposed to each other, and more particularly to a control system that controls a transmission ratio of the toroidal continuously variable transmission to match a target transmission ratio.
2. Description of Related Art
A known example of such a control system for the toroidal continuously variable transmission is disclosed in Japanese Patent Application Laid-Open No. 8-233085.
This control system includes a position detection means for detecting a positional displacement y in the trunnion axis direction of power rollers supported on the trunnion, and an angle detection means for detecting a tilt angle .phi. of the power rollers about the trunnion axis. The control system calculates an actual transmission ratio i of the toroidal continuously variable transmission from the tilt angle .phi. detected by the angle detection means and, based on a deviation between the transmission ratio i and a target transmission ratio ic, calculates a target positional displacement yc of the power rollers in the trunnion axis direction. A feedback control is performed to make zero the deviation between the transmission ratio i and the target transmission ratio ic by displacing the trunnions in the trunnion axis direction by hydraulic actuators, to match the positional displacement y of the power rollers in the trunnion axis direction detected by the position detection means to the target positional displacement yc.
FIG. 7 shows a block diagram of the conventional control system for the toroidal continuously variable transmission.
The conventional control system has following problems; because the position detection means and the angle detection means must be installed at either end of the trunnion, the size of the trunnion in its axial direction increases, resulting in deteriorating the mountable tolerance of the transmission in a vehicle body or reducing the ground clearance or minimum height of the vehicle body from the ground. Further, because the position detection means and the angle detection means generally cannot be installed in oil, it is necessary to install these detecting means outside the transmission case or to prevent oil from contact with the means by oil seals, which in turn complicates the arrangement of the control system.
Further, an axial force 2Ft in the trunnion axis direction that is applied from an input disk and an output disk to the power rollers has a great effect as external disturbance acting on the feedback control of the positional displacement y of the power rollers. Thus, with the conventional method that first detects a positional displacement y and then calculates its deviation from a target positional displacement yc to operate the hydraulic actuators, not only a sufficient response can not be obtained, but there is a possibility hunting may be generated in the determinaton of the transmission ratio.